1. Field of the Invention
The present invention is directed to scanning probe microscopes (SPMs), including atomic force microscopes (AFMs), and more particularly, to a mode of AFM operation that provides force control at high speed, low tip-sample interaction forces and high resolution.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 1. An AFM 10 employs a probe device 12 including a probe 17 having a cantilever 15. A scanner 24 generates relative motion between the probe 17 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings et al. U.S. Pat. No. 5,412,980.
Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
In a common configuration, probe 17 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 17 to oscillate at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever 15. Probe 17 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 17 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 17 but may be formed integrally with the cantilever 15 of probe 17 as part of a self-actuated cantilever/probe.
Often, a selected probe 17 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 17, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 17, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are processed by a signal processing block 28 (e.g., to determine the RMS deflection of probe 17). The interaction signal (e.g., deflection) is then transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 17. In general, controller 20 determines an error at Block 30, then generates control signals (e.g., using a PI gain control Block 32) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 17. The control signals are typically amplified by a high voltage amplifier 34 prior to, for example, driving scanner 24. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used. Controller 20 is also referred to generally as feedback where the control effort is to maintain a constant target value defined by setpoint.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform data manipulation operations such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving the sample and/or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one practical mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe, or harmonic thereof. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force.” i.e., the force resulting from tip/sample interaction, typically by controlling tip-sample separation. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
As with most measuring devices, AFMs often require a trade-off between resolution and acquisition speed. That is, some currently available AFMs can scan a surface with sub-angstrom resolution. These scanners are capable of scanning only relatively small sample areas, and even then, at only relatively low scan rates. Traditional commercial AFMs usually require a total scan time typically taking several minutes to cover an area of several microns at high resolution (e.g. 512×512 pixels) and low tracking force. The practical limit of AFM scan speed is a result of the maximum speed at which the AFM can be scanned while maintaining a tracking force that is low enough not to damage or cause minimal damage to the tip and/or sample. Great strides have been made in this area in which SPM has achieved video scan rates with high resolution for small samples and small scan sizes.
Nonetheless, given current limitations associated with known modes of operation, including both TappingMode™ AFM and contact mode, improvements have been desired. Again, in contact mode, lateral scanning of the tip creates large forces between the tip and sample that can compromise both. And when imaging soft samples such as biological samples and polymers, the surface can be destroyed, rendering the measurement useless, or at least deformed severely, thereby significantly compromising resolution. Note that “imaging” is used herein to indicate obtaining SPM data at multiple points of a sample surface, typically by providing relative scanning motion between the sample and probe and correspondingly interacting the sample and probe.
TappingMode™ AFM is a lower force technique and is the most widely used mode of AFM operation to map sample surfaces, especially for delicate samples. The typical force of the tip on the sample is about a few nN to tens of nN. Again, by oscillating the tip, rather than dragging the tip, the shear forces are minimized. That said, TappingMode™ AFM suffers from a drawback in that it is difficult to control the normal force acting on the sample surface. The user typically tries to select a setpoint that is only a small variation from the free air deflection/amplitude of the probe in order to minimize tip-sample interaction forces to get the best reproduction of the sample profile. The dilemma, especially for soft samples, is that if the imaging force is too low, the tip will not track the sample properly (i.e., maintain interaction with the sample during the scan), while if too high, damage/deformation of the sample may lead to an image that does not accurately reflect surface topography. Overall, the better this force can be controlled (i.e., the lower it can be maintained) the less chance of sample and/or tip damage, and thus resolution can be improved.
A review of the tip-sample forces in each of these modes provides insight in to the limitations of each. When a probe interacts with the surface through TappingMode™ AFM or Jumping Mode™ (see, e.g., U.S. Pat. Nos. 5,229,606, 5,266,801 and 5,415,027, the entirety of which are incorporated by reference herein), the tip touches the surface periodically. FIG. 2A illustrates the physical process within one period “T” of the tip motion. FIG. 2A shows tip trajectory in reference to the sample surface position. FIG. 2B shows the corresponding interaction force at the same time for tip trajectory at various positions. At the peak positions Amax, the tip is farthest from the sample surface and not interacting with the sample. As the tip continues to move down toward the horizontal axis (zero tip-sample separation) it will experience a near-field Van der Waals force, Fa_vdw, causing the tip to snap into contact with the sample through Van der Waals attraction. After touching the sample, the tip remains in repulsive interaction for time zone δT. During this time, the tip is continuously contacting the sample. The positions below zero represent that the tip may have deformed the sample, causing its position to be shown below the sample surface.
As the tip departs the surface after δT, an attractive force will develop a capillary meniscus, exhibiting a maximum adhesion force Fa_max right before the meniscus is broken away. The tip then enters into a non-interactive region and continues to a maximum departure position.
In the interaction free zone, when the probe is farther from the surface, the interaction force is zero or sufficiently near zero to form a baseline, as indicated in FIG. 2B. In FIG. 2B, the force above the horizontal axis is repulsive while those points below the horizontal axis represent a net attractive or adhesive force. The maximum repulsive force Fr_max usually corresponds to the lowest or smallest tip position or separation relative to the sample surface.
In prior known modes disclosed in TappingMode™ AFM and JumpingMode™ AFM, the amplitude Amax or RMS of the tip oscillation amplitude is used as the feedback control parameter. An example of such feedback control apparatus is shown in FIG. 1.
In conventional control, typically implemented using a gain control feedback loop, positioning actuators and a cantilever response detection component (quadrant photodetector, for example), the AFM uses detected probe deflection or an RMS signal corresponding to cantilever (i.e., probe) motion as an indication of the tip-surface interaction and uses the feedback loop to maintain constant or RMS deflection.
Yet a major limitation of conventional AFM is its inability to acquire quantitative mechanical property information simultaneously with the high-resolution imaging. AFM has been primarily focused on topographical imaging. Little progress has been made in achieving quantitative mechanical mapping, including elasticity, plasticity, and work of adhesion.
Moreover, TappingMode™ control uses amplitude or phase of the measured deflection signal to control tip-surface interaction using feedback. Notably, both amplitude and phase are average properties of the probe/tip oscillation using at least one cycle of interaction. More specifically, the average pertains to probe/sample interactions occurring in all the positions in the tip trajectory (FIG. 2). Therefore, there is no possibility for the control feedback to be based on substantially instantaneous tip-sample interaction. Note that instantaneous interaction here refers to any point (for example, within two microseconds) of interaction in FIG. 2B (discussed further below).
In addition, it is important to note that TappingMode™ AFM was created to overcome what is known as the stick-in condition that occurs when probe touches the sample intermittently. As the probe touches the sample, capillary force will tend to catch the tip and prevent it from releasing. The amplitude of probe oscillation in TappingMode™ will drop to zero, thereby causing feedback oscillation. This problem was overcome when using TappingMode™ by using probes having a certain stiffness, usually 10 N/m (Newton/meter) to 60 N/m, with a nominal value of 40 N/m, while operating the TappingMode™ AFM at an oscillation amplitude higher than about 10 nm peak-to-peak. Under these conditions, as the probe touches surface, the kinetic energy of the tapping probe coverts to enough static elastic energy to overcome the capillary force, assuring steady amplitude in each cycle. One drawback of this mode is that the kinetic energy stored in the probe is also proportional to the cantilever spring constant. When employing a lower spring constant cantilever, such as 1 N/m, TappingMode™ is impossible when measuring many materials because the cantilever cannot overcome the capillary adhesion forces using its own resonance oscillation energy. Consequently, most TappingMode™ applications are only possible when one uses a stiff cantilever generally known in the art as a lever.
In an alternate mode of operating an SPM, known as the pulsed-force mode or PFM (see, e.g., U.S. Pat. No. 6,880,386 and U.S. Pat. No. 7,129,486), the amplitude of the oscillation of the probe is adjusted so the tip goes in and out of contact during each cycle. In this mode, control is provided by monitoring tip-sample interaction forces. It operates based on properties associated with a force curve, another common measurement made in the AFM field to measure material properties at a particular location. Force measurements are common, and can be mapped over an entire sample to create what is known as a force-volume image.
In PFM, by analyzing the shape of the force-distance curve, and using the data to control the forces acting between the tip and the sample, the amount of data acquired is lessened compared to other modes of SPM operation. Importantly, PFM typically needs to operate at Fr_i (discussed below) or the peak pulse force, which substantially exceeds the adhesion induced deflection, as well as coupling induced deflections. As a result, a high repulsive force is needed as a control reference. Such high force could damage the sample or the tip, and thus prevent acquisition of high resolution images. Moreover, PFM has other limitations, particularly with respect to operating speed and resolution limitations, and thus, though it has been implemented to image soft samples, it has not been more widely adopted for all types of AFM imaging applications. In addition, when imaging in a fluid environment provide further challenge to PFM since viscous force in fluid produces large deflection even when the cantilever probe is not interacting with the sample.
More particularly, a main reason why imaging speed is limited in standard PFM AFM is illustrated in FIG. 2C. FIG. 2C is a graph of tip-sample interaction force versus time. The interaction force is plotted as snap-to-contact at “A”, at which point repulsive force (sample on tip) initiates at “B.” Peak repulsive force occurs at about “C” as adhesive forces pull on the tip until about point “D”, the point at which the tip releases from the sample. Point E represents the deflection peak of the cantilever probe when it departs from the sample. Points C and E both present themselves as a peak in the deflection signal. In order to assure that feedback controls tip-sample interaction properly, the value of C should exceed E. In yet another constraint in PFM, a certain ringdown period (cycles of the probe oscillation at its resonance frequency) is required before it is possible to determine the baseline force needed to continue the scan. It is this waiting for the cantilever to “ringdown” (a free decay process, as in TappingMode™) that limits the modulation frequency, and thus scan speed. More particularly, modulation frequency is significantly less than the probe resonance frequency (for example, a fifth or more below the probe resonance frequency).